Black young men who have sex with men (BYMSM) are at high risk for contracting and transmitting HIV, and represent a priority population for developing effective interventions. Within the House Ball community (HBC), a clandestine subculture of the Black gay community, HIV is highly stigmatized. This stigma, coupled with high HIV prevalence rates and elevated levels of undiagnosed/untreated HIV infection, places BYMSM in HBCs at incredible risk for HIV infection. Community-level interventions that target social norms, behavior, and stigma in the HBC are sorely needed in order to make a broader impact among BYMSM. Building on prior work in the community, this study proposes to launch an innovative Effectiveness-Implementation Hybrid Type 2 trial to examine the effectiveness and implementation of a community-level HIV prevention intervention (called POSSE) based on popular opinion leader (POL) models across two cities with similar HBCs, Chicago and Philadelphia. The proposed study has two specific aims: 1) To determine the effectiveness of a POL intervention, POSSE, to decrease sexual risk behavior (UAI), STIs and HIV stigma among BYMSM in the HBC; and 2) To evaluate the processes, strategies, barriers and facilitators for the implementation of POSSE delivered across two distinct metropolitan areas with high HIV prevalence among BYMSM and similar House Ball Communities. To examine these aims, we will first use social network strategies to identify, screen, and recruit POLs (n=75) from the Philadelphia HBC. We will then gather baseline behavioral and biological (HIV/STI testing) data in both Chicago and Philadelphia (n=100 BYMSM per city) and train the Philadelphia POLs to implement the POSSE intervention. We will conduct an implementation-focused process evaluation to assess fidelity to the POSSE intervention; number, content, spread, and acceptability of risk reduction messages; age appropriateness of the messages; acceptability of the overall intervention; and barriers/facilitators to implementation. Next, we will complete follo-up assessments every 6 months post- intervention in both cities (n=100 BYMSM per city at each assessment point). We will also assess behavior change in those delivering the intervention (POLs) at each assessment point. We will conduct implementation- focused qualitative interviews with POLs (n=15) and community participants (n=30) from Philadelphia. Last, we will implement POSSE in Chicago by repeating the steps outlined above, continuing assessments in both cities, and conducting qualitative interviews in Chicago. This study is significant and innovative in that it proposes to simultaneously evaluate the effectiveness and implementation of a community-level intervention for BYMSM, a group disproportionately impacted by the HIV epidemic, in order to expeditiously move effective public health programming into the community.